Method for pneumatic classification and a pneumatic classifier

ABSTRACT

The pneumatic classifier as disclosed includes a main body, means to produce air circulation in the body, means to feed material to be classified thereto, cylone means to collect the final fine product, hopper means to collect the final coarse product, the classifier including an ejector as primary dispersion means located within the classifier body on a selected reference level, horizontal centrifugal classifier means located above the said reference level, and a secondary dispersion means located below the said reference level, the classifier being arranged to carry out the method steps of: 1. PRIMARY DISPERSION OF THE FEED MATERIAL INTO AN AIR SUSPENSION IN THE PRIMARY DISPERSION MEANS BY APPLYING THE EJECTION PRINCIPLE, 2. SEPARATION OF THE FRACTION RETAINED IN THE CREATED SUSPENSION IN A HORIZONTAL CENTRIFUGAL FIELD ON A HIGHER ELEVATION INTO A FINAL FINE PRODUCT AND A COARSER MIDDLING PRODUCT WHICH IS RETURNED DOWNWARD BACK INTO THE AIR SUSPENSION PRODUCED IN THE SAID PRIMARY DISPERSION MEANS, OR IS REMOVED AS A SEPARATE MIDDLING PRODUCT, AND 3. SECONDARY DISPERSION OF THE FRACTION SETTLING BY GRAVITY FROM THE SAID AIR SUSPENSION IN THE SECONDARY DISPERSION MEANS ON A LOWER ELEVATION BY MEANS OF A DESIRED PART OF THE TOTAL AIR STREAM USED FOR CLASSIFICATION INTO A FINAL COARSE PRODUCT AND A FINER MIDDLING PRODUCT WHICH IS DIRECTED UPWARD WITH THE AIR MEDIUM DIRECTLY INTO THE SAID HORIZONTAL CENTRIFUGAL CLASSIFIER.

United States Patent Hukki [451 Apr. 15, 1975 METHOD FOR PNEUMATIC CLASSIFICATION AND A PNEUMATIC CLASSIFIER [75] Inventor: Risto Tapani Hulrlti, Otaniemi,

Finland [73] Assignee: Kennedy Van Satin Corporation,

. Danville, Pa.

22 Filed: Sept. 15, 1972- [21 Appl. No.: 289,270

[30] Foreign Application Priority Data Sept. 28, 1971 Finland 2683/71 [52] US. Cl. 209/12; 209/135; 209/144; 209/143; 209/146 [51] Int. Cl B07b 9/02 [58] Fieldjof Search 209/133-139 R, 209/146, 12, 144, 143

[56] References Cited UNITED STATES PATENTS 1,962,668 6/1934 Olney 209/136 2,214,434 9/1940 Nelms 209/135 2,381,954 8/1945 Hardinge 209/144 2,767,840 10/1956 Dobson et a1 209/144 2,931,581 4/1960 Lykken et a1 209/144 X 2,941,667 6/1960 Hilgartner et a1 209/134 3,010,576 11/1961 Harte et a1. 209/35 3,017,993 l/l962 MacPherson et al. 209/144 3,738,483 6/1973 Mackenzie 209/137 X FOREIGN PATENTS OR APPLICATIONS 552,104 4/1957 Belgium 209/136 Primary Examiner-Frank W. Lutter Assistant ExaminerRalph .1. Hill 57 ABSTRACT The pneumatic classifier as disclosed includes a main body, means to produce alt circulation in the body, means to feed material to be classified thereto, cylone means to collect the final fine product, hopper means to collect the final coarse product, the classifier including an-ejector as primary dispersion means located within the classifier body on a selected reference level, horizontal centrifugal classifier means 'located above the said reference level, and a secondary dispersion means located below the said reference level, the classifier being arranged to carry out the method steps of:

1. primary dispersion of the feed material into an air suspension in the primary dispersion means by applying the ejection principle,

2. separation of the fraction retained in the created suspension in a horizontal centrifugal field on a higher elevation into a final fine product and a coarser middling product which is returned downward back into the air suspension produced in the said primary dispersion means, or is removed as a separate middling product, and

3. secondary dispersion of the fraction settling by gravity from the said air suspension in the secondary dispersion means on a lower elevation by means of a desired part of the total air stream used for classification into a final coarse product and a finer middling product which is directed upward with the air medium directly into the said horizontal centrifugal classifier.

7 Claims, 3 Drawing Figures RITE-HEB AFR I 5 25. 5

sum 2 or 3 FIG. 2

PATENTEEAPR 1 5 {9Y5 SEEEET 3 FIG. 3

METHOD FOR PNEUMATIC CLASSIFICATION AND A PNEUMATIC CLASSIFIER This invention relates toamethod for continuous pneumatic classification of finely divided solids and to the respective pneumatic classification apparatus.

In practice. pneumatic classification of finely divided solids is commonly performed in centrifugal classifiers. The conventional centrifugal classifier is of vertical construction. The horizontal centrifugal classifier is also known.

In recent years improvements in centrifugal classifiers have been especially directed to means to improve their sharpness of separation. Numerous inventions have been made for the purpose of cleaning further the sand product of the fines retained in it.

The present invention concerns an improved method of classification where the process is distinctly subdivided into its basic components. where each one of them iscarried out under essentially independent conditions with an emphasis on utmost perfectness, and where the said components together form a continuously operating wholeness. This is obtained by primary dispersion of the feed material into an air suspension in the primary dispersion step by applying the ejection principle by separating the fraction retained in the created suspension in a horizontal centrifugal field on a higher elevation into a final fine product and a coarser middling product which is returned downward back into the air suspension produced in the said primary dispersion step. or is removed as a separate middling product, and

- by secondary dispersion of the fraction settling by gravity from the said air suspension in the secondary dispersion step on a lower elevation by means of a desired part of the total air stream used for classification into a final coarse product and a finer middling product which is directed upward with the air medium directly into the said horizontal centrifugal field.

While the indicated two dispersion steps of the material to be classified seem to be quite adequate for most operations, it should be obvious that their number may be increased if desirable by following the described pattern.

The centrifugal classification step can also be performed as a multiple step process in such a way that the fine product suspension of a preceeding step forms the feed suspension to a succeeding step performed in a similar horizontal centrifugal field placed stepwise on a higher elevation and operated under stepwise higher gee-value.

This invention relates further to an improved pneu- 'matic classification apparatus operating in accordance with the said principles. The apparatus is characterized by primary dispersion means constructed on the ejection principle. placed within the classifier housing in such a way that the said primary dispersion means on the whole is situated approximately on the reference level formed by the crossing line of air inlet channel and feed material inlet channel, by a horizontal centrifugal classifier or a corresponding series classifier placed on an elevation above the level of the primary dispersion means named above. and a by secondary dispersion means constructed essentially on a similar ejection principle and placed on an elevation below the level of the said primary dispersion means.

As indicated above. the most essential features of this invention are the steps taken and the means applied for utmost possible dispersion of the feed material. Thoroughly dispersed feed powders only will lead to clean separations. Thus sufficiently powerful dispersion means are an absolute necessity for a sound pneumatic classification process and apparatus. The entire action of the said means are according to the present invention directed to this basic task which in most of the earlier classifiers has not received enough attention. The dispersion means in the present invention are constructed to include an ejector having no moving parts an ejector having no moving parts and further invigorated by stationary counter bars an ejector with no moving parts invigorated by a mechanical part rotated at a high speed.

The centrifugal classifier pro per is preferably built to include stationary parts. only. The other alternative includes within it drum-like means rotated about a horizontal axis. The periphery of the said drum can be constructed of a number of similar. apart of each other placed rods. or ribs. parallel to the shaft of the drum. The said ribs may alsohave' an isosceles angular shape or two equal sides and pointing in the direction of rotation.

To make the classifier on the whole a handy and simple unit it is advantageous that the classifier housing. the primary and the secondary dispersion means within it. the centrifugal classifier. the fine product cyclone. the connecting air tube and the blower form one unified apparatus operating in a closed circuit.

The basic construction and the operating principle of .the pneumatic classifier constructed in accordance with the present invention are explained more in detail by referring to the accompanying drawings of which FIG. 1 shows a schematic vertical section of a classifier which disregarding the blower does not include moving parts,

FIG. 2 a corresponding section of a classifier of another construction with no moving inside parts. and

FIG. 3 a similar section of a classifier where both the dispersion means and the centrifugal classifier include a moving part. each.

As shown by HO. 1 the classification apparatus includes as a main body box-like housing 1, feeder means 2 for material powder to be classified. cyclone collector means 3 for the fine product. collecting hopper 4 for the final coarse product and air tube 5 connecting cyclone 3 to blower 6. Primary dispersion means 7 has been placed within the front upper part of the classifier box and centrifugal classifier 8' having a horizontal axis on top of the rear section of the box. The dispersion meansshown in FlG. 1 includes stationary parts, only. the most essential ones being air inlet channel 9, feed material inlet channel 10, mixing channel 11 for the air and feed material, the said parts together forming an ejector. The primary dispersion means is situated approximately on the reference level formed by the crossing line of air inlet channel 9 and feed material inlet channel 10. The ejector opens to primary dispersion space 12 which is furnished with a set of flow guiding plates 13. The entrance to air channel 9 is provided with hinged flow control baffle l4.

In the apparatus shown in FIG. 1, centrifugal classitier 8 is constructed of stationary parts. only, the most essential ones being wide inlet opening or channel 15, circular discharge opening 16 for the suspension carrying the fine product and relatively narrow downward leading discharge slot 17 for the coarser middling product. Opening 16 is connected tangentially to the upper section of cyclone 3.

Secondary dispersion means 18 include in FIG. 1 air channel 19, funnel shaped feeder channel 20, secondary dispersion space 21 and flow channel 22 leading directly to centrifugal classifier 8. As can be seen. the primary and secondary dispersion means and the primary and secondary dispersion spaces are clearly separated from each other.

According to FIG. 1, classifier housing 1, primary dispersion means 7, secondary dispersion means 18, centrifugal classifier 8, fine product cyclone 3, connecting air tube and blower 6 form a unified system operating in closed circuit. Branch pipe 23 attached to main air tube 5 is. connected to an outside dust removal system not, shown in the picture.

The classifier shown in FIG. 2 deviates from that shown FIG. I in two essential respects. only. To primary dispersion means 7 is here added a series of stationary countergrids 24. Furthermore. the centrifugal classifier is now of two stage construction and includes primary centrifugal classifier 8 and secondary centrifugal classifier 25 of a smaller size. Discharge opening 16 in classifier 8 forms the entrance to inlet channel 26 feeding classifier 25. Circular discharge opening 27 is now connected tangentially to cyclone 3. Downward directed discharge slot 28 for the respective middling product leads either to primary dispersion space 12 or to a separate collecting hopper (not shown).

The classifier shown in FIG. 3 differs from those already explained in that dispersion means 7 and centrifugal classifier 8 include new a moving part. each. In FIG. 3 dispersion drum 29 rotated about a horizontal axis has been placed at the mounth of the primary dispersion means (primary ejector) 7. The drum periphery consists of evenly spaced rods. The drum is rotated at a desired speed by an outside mechanism. To further improve dispersion, stationary grid 30 has been placed opposite to drum 29.

Centrifugal classifier 8 shown in FIG. 3 illustrates one possible way of construction. In it, drum 31 is rotated about a horizontal axis at a desired speed by an outside mechanism. The drum is preferably designed purely for sizing action and it should not cause marked inward or outward flow of air when rotated alone. Instead of conventional rods. the drum periphery consists here of similar, evenly spaced ribs. apart of each other and parallel to the shaft of the drum. To reach the recommended neutral effect each rib preferably has an isoceles angular shape or have two equal sides and pointing in the direction of rotation, as shown in FIG. 3.

The classification apparatus constructed in accordance with the principles of this invention works as follows:

Referring to FIG. 1 powder to be classified is introduced via feeder 2 at an angle determined by channel 10 into primary dispersion means 7 where the powder gets into the fast moving air jet lamella discharging from channel 9, and becomes mixed with the air in channel 11 in accordance with the principles of ejector operation. The dispersed air suspension spreads out LII into primary dispersion space 12 in the classifier box. The fraction of solids retained in suspension flows at a high velocity and guided by plates 13 via inlet opening 15 into centrifugal classifier 8 where. the solids are sized according to the well knownprinciples in such a way that the finest size fractions followthe medium into cyclone 3 while the coarser size fractions with some fines discharge as a middling product via discharge slot 17 downward into primary dispersion space 12. The fines still included in the middling material have now a new chance in getting back to classifier 8. Those particles not staying in suspension settle downward by gravity into funnel shaped space 20. The said particles are redispersed by secondary dispersion means 18 resembling here a simplified ejector. Air for this ejector is introduced via secondary air channel 19. The resulting dispersion spreads out into secondary dispersion space 21 wherefrom the suspended fines follow the medium upward via channel 22 directly into centrifugal classifier 8 while the cleaned final coarse product settles downward and collects into hopper 4. The classified fine product separates from the medium in cyclone 3. Both products are continuously discharged from the apparatus in any proper way.

The medium with some of the very finest dust returns from cyclone 3 via tube 5 into blower 6 and further in a closed circuit back into the classification apparatus proper. As shown by FIG. I, feed material to be processed is introduced here via open feeding means 2. In such a case, backblowing of some dust via feeder 2 is prevented by removing a fraction of the circulating air stream discharging from cyclone 3 into an outside dust removal system via branch pipe 23. This causes a slight inward flow of air via feeder 2 with the feed material. By using a sealed feeding system of any conventional type (not shown) instead of the open one the need for an outside dust collector can be eliminated.

The basic principle of operation of the classifier shown in FIG. 2 is essentially the same as that already described. Stationary series of countergrids 24 are used to invigorate the dispersion action of primary dispersion means 7. The two step centrifugal classifier leads to a very unique method of sizing. As shown, the air suspension discharging from classifier 8 via opening 16 forms as such and without any further effort a well dis persed feed suspension to secondary centrifugal classifier 25 to which it is introduced via inlet channel 26. The reclassified fines follow the medium via opening 27 into cyclone 3 where the solids separate from the medium as already described. The respective coarser middling product proceeds downward via channel 28 either into primary dispersion space 12 or into a hopper (not shown) to be withdrawn as a separate product. It is natural that the centrifugal series classifier can include any number of steps following the indicated flow pattern. The outstanding advantage of such an arrangement is that reclassification of the dispersed-suspension created in a preceeding centrifugal unit takes place instantaneously in a succeeding centrifugal unit operated under a susbstantially higher gee-value. The higher geevalue is obtained by decreasing the active volume of the succeeding centrifugal unit in comparison with the preceeding unit by decreasing the active radius. or its depth in the directionof the axis, or both.

In case shown in FIG. 3 the powder-air mixture discharges from ejector 7 onto dispersion drum 29 rotated at a desired speed. Rotation of the drum causes strong mechanical attrition and fast moving whirls improving dispersion of the powder. This action can be further intensified by a series of stationary rods in countergrid 30 against which the streams must strike. In the centrifugal classification unit shown in FIG. 3 the size of separation is determined by the top particle size capable of following the medium between the ribs in drum 31. The coarser particles when hit by the outer rims of the ribs bounce back into the space surrounding the drum wherefrom they are discharged with the rest of the oversized particles via channel 17 into primary dispersion space 12 as already described.

As the apparatus according to the present invention is very compact in size, the units can be easily arranged in parallel or in series. For normal industrial applications no more than two units in series seem to be necessary.

The essential control operations for a given apparatus include regulation of the total air volume, regulation between the fractional volumes used for primary and secondary dispersion. and regulation of the speed of rotation of the dispersion drum and of the centrifugal classifier drum in cases where they are included within the apparatus.

The method for pneumatic classification and the corresponding pneumatic classifier according to this invention can be used in classification sizing of all such products that can be treated in existing pneumatic classifiers of any conventional type. The method and apparatus described in this specification seems to be especially suitable for production of extremely fine products such as e.g. so-called super cement, filling materials such as talc and kaoline used in paper industry, and for the entire wide field of products characterized by the name micropowders.

The ability of the classification apparatus according to this invention to accomplish separations of unusual characteristics will be demonstrated with a reference to Tables I and 2 giving detailed data on tests carried out on pilot plant scale.

The data reported in Table 1 were obtained in a pilot plant unit substantially as shown in FIG. I. The dimensions of classifier box 1 were 75 cm (height) X 60 cm (Width) X 20 cm (depth). The effective volume of cen- 5 trifugal classifier unit 8 on top of the box was about 12 dm. With exception of blower 6, no moving parts were used.

The data show that by a single run from 400 up to 1,500 kilos of fine product having the fineness of 95% 36 microns to 95% -46 microns. resp.. were obtained as the cyclone product. By rerunning the respective coarse product a second time, additional fine material was obtained as indicated. It should be emphasized that 15 the said separations were obtained in a centrifugal unit having a volume of 12 liters, only. Furthermore. the separations were obtained at energy consumption figures lower than never before reported in respective cases. As indicated. the dust loads carried in suspensions entering the cyclone were exceptionally high. The

figures indicating circulating loads show that in practical operations no more than two complete units in series are needed for top separations.

The data reported in Table 2 were obtained in a pilot sumption figures. Middlings from classifier 25 were withdrawn as separate products; the respective data are given in Table 2. Summation offines and middling gives line F M which represents the corresponding one step centrifugal processing similar to that reported in Table 1. In some respects the corresponding data reported in Table 2 are even superior.

Table 1 Experimental data on classification tests with ground cement clinker in classifier No. 1 (20 cm deep pilot plant unit substantially as shown in FIG. 1

Test Feed Fine product kWh/t Dust Circ. No, t/h t/h "/1 of 2 -37 -53 74 105 95 7c of load load original ;I. p. p. p. x p. fine feed prod. kg/m" 71 1 1st run 1.89 0.421 22.3. 93.6 95.2 99.0 99.8 99.9 36 1.75 1.7 47

v 0.767 40.6 93.1 94.7 98.9 99.7 99.9 37 1.92 1.6 2 15! run 2.78 0.772 27.8 91.2 93.2 98.2 99.3 99.9 41 1.06 2.0 260 2nd 0.528 19.0 88.6 91.4 98.0 99.3 99.8 44 1.54 1.5 114 1.300 46.8 90.1 92.5 98.1 99.3 99.9 42 1.26 1.9 3 1st run 3.14 1.005 32.0 90.8 92.8 98.0 99.1 99.8 42 0.91 2.2 213 2nd 0.597 19.0 87.6 90.6 97.6 99.1 99.8 46 1.53 1.3 96

1.602 51.0 89.6 92.0 97.9 99.1 99.8 43 5 1.14 1.9 4 1st run 4.48 1.257 36.1 88.4 90.8 96.2 98.4 99.4 48 0.91 1.8 177 2nd 0.853 24.5 85.6 88.8 95.8 98.4 99.2 50 1.34 1.2 66

2.110 60.6 87.3 90.0 96.0 98.4 99.3 49 1.08 1.6 5 1st run 4.42 1.494 33.8 88.2 90.8 97.2 98.8 99.6 46 0.68 2.7 196 2nd 0.911 20.6 87.2 90.0 97.0 98.7 99.5 47 1.11 1.6 84

The original cement clinker has been ground to a fineness of 87 '/l 74 a and 64 '4 37 a. 0.5 uftriethanolamine as a dispersant was added to the mill.

Table 2 Experimental data on classification tests with ground cement clinker in classifier No. 2 (20 cm deep pilot plant unit substantially as shown in FIG. 2)

Test Feed Product t/h of l5 p. p. 32 p' 95 7! kWh/t Dust Circ. Specific No. t/h original x a of load load surface ed fine area product kg/m" 7r cm'-'/g 6 1.15 Fines 0.238 1.7 78.4 88.5 98.8 3.60 0.6 334 4900 Middling 0.219 19.0 45.8 65.3 93.8 34 2190 F M 0.457 39.7 62.8 77.4 96.4 29 1.88 1.1 152 3600 Sands 0.696 60.3 1 1.1 16.6 34.2 970 7 2.26 Fines 0.347 15.3 80.3 89.9 98.8 24 2.47 0.9 552 Middling 0.332 14.7 46.9 64.2 90.9 36 F M 0.67) 30.0 64.0 77.3 94.9 32 1.26 1.7 233 Sands 1.584 70.0 17.7 24.4 43.8 8 2.78 Fines 0.388 14.0 79.2 89.2 98.2 25 2.21 1.0 616' Middling 0.344 12.4 46.7 64.1 90.1 37 F M 0.732 26.4 63.9 77.4 94.4 33 1.17 1.8 280 Sands 2.047 73.6 21.7 29.5 49.5 9 3.37 Fines 0.424 12.6 77.1 86.7 98.3 26 2.02 1.1 695 Middling 0.330 9.8 47.2 64.2 902 37 F M 0.754 22.4 64.0 76.9 94.8 32' 1.14 1.9 347 Sands 2.615 77.6 21.8 29.5 47.3

The original cement clinker as ground in the presence of 0.5 of triethanolamine to a fineness of 88 'i 74 p. 59 i 32 u and 32 i l5 u.

I claim:

1. A method of pneumatically classifying finely divided solid materials comprising the steps of:

a. delivering the material to be classified into an air stream to form a suspension ejecting the resulting suspension into a primary dispersion space wherein coarse material settles out by gravity,

b. passing the air suspension with the finer material into a centrifugal classifying field wherein a final fine product is recovered in the air stream and a coarser middling product is separated therefrom,

c. passing the settled coarse material from the primary dispersion space into an air stream to form a suspension, ejecting the resulting suspension intoa secondary dispersion space wherein a final coarse product settles out by gravity from the air suspension containing middling material, and

d. passing the resulting air suspension into the centrifugal field and mingling it with the air suspension from the primary dispersion space.

2. A pneumatic classifier for classifying finely divided material including a housing, means providing separate primary and secondary dispersion spaces in the housing. primary and secondary dispersion means discharging respectively into the dispersion spaces, means for delivering a stream of air to each dispersion means,

means for supplying finely divided material to be classified into the stream of air in the primary dispersionmeans for delivery into the primary dispersion space. a centrifugal classifier, means for conducting air and material suspended therein from the primary dispersion space as a stream into the centrifugal classifier for the separation of a middling product from the fine material carried by the air stream, means for supplying coarse material settled out in the primary dispersion space into the air stream in the secondary dispersion means for delivery to the secondary dispersion space. means, for conducting the air dispersion from which coarse material has settled out in the secondary dispersion space into the centrifugal classifier to mingle with the air suspension delivered thereto from the primary dispersion space. and means providing a discharge outlet structure in the centrifugal classifier for the dispersion of air and fine material circulating therein.

3. A classifier as claimed in claim 2, including means for conducting the middling product from the centrifugal classifier into the dispersion in the primary dispersion space.

4. A classifier as claimed in claim 2, including a horizontally rotatable dispersion drum means located in the primary dispersion space adjacent to the outlet of the primary dispersion means.

' 5. A classifier as claimed in claim 2, including a rotatable drum like means located in the centrifugal classifier in axial alignment with the discharge outlet and extending around its periphery.

6. A classifier as claimed in claimS, wherein the periphery of the drum like means is comprised of spaced ribs parallel to the axis of the drum.

7. A classifier as claimed in claim 5, wherein the drum-like means comprises spaced ribs parallel to the axis of the drum each rib having an isosceles angular shape in cross section pointing in the direction of rotation of the drum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 7 9 Dated 1 April 5, 75 O Inventor(s) Risto Tapani Hukki It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Table 1, last column in Test No. l, lst run "47 should read -347-;

Table 2, Test No. 6, 6th column "78.4" should read O -70.4-.

Sign! and Scalcd this second Day of December 1975 [SEAL] AIICSI.

' RUTH C. MASON C MARSHALL DAN" Arresting Officer Commissioner ofPatehls and Tmdenlarks FORM PC4050 (10-69) USCOMM-DC 60376-5 69 U.S. GOVERNMENT PRINTING OFFICE: O I 

1. A method of pneumatically classifying finely divided solid materials comprising the steps of: a. delivering the material to be classified into an air stream to form a suspension ejecting the resulting suspension into a primary dispersion space wherein coarse material settles out by gravity, b. passing the air suspension with the finer material into a centrifugal classifying field wherein a final fine product is recovered in the air stream and a coarser middling product is separated therefrom, c. passing the settled coarse material from the primary dispersion space into an air stream to form a suspension, ejecting the resulting suspension into a secondary dispersion space wherein a final coarse product settles out by gravity from the air suspension containing middling material, and d. passing the resulting air suspension into the centrifugal field and mingling it with the air suspension from the primary dispersion space.
 2. A pneumatic classifier for classifying finely divided material including a housing, means providing separate primary and secondary dispersion spaces in the housing, primary and secondary dispersion means discharging respectively into the dispersion spaces, means for delivering a stream of air to each dispersion means, means for supplying finely divided material to be classified into the stream of air in the primary dispersion-means for delivery into the primary dispersion space, a centrifugal classifier, means for conducting air and material suspended therein from the primary dispersion space as a stream into the centrifugal classifier for the separation of a middling product from the fine material carried by the air stream, means for supplying coarse material settled out in the primary dispersion space into the air stream in the secondary dispersion means for delivery to the secondary dispersion space, means for conducting the air dispersion from which coarse material has settled out in the secondary dispersion space into the centrifugal classifier to mingle with the air suspension delivered thereto from the primary dispersion space, and means providing a discharge outlet structure in the centrifugal classifier for the dispersion of air and fine material circulating therein.
 3. A classifier as claimed in claim 2, including means for conducting the middling product from the centrifugal classifier into the dispersion in the primary dispersion space.
 4. A classifier as claimed in claim 2, including a horizontally rotatable dispersion drum means located in the primary dispersion space adjacent to the outlet of the primary dispersion means.
 5. A classifier as claimed in claim 2, including a rotatable drum like means located in the centrifugal classifier in axial alignment with the discharge outlet and extending around its periphery.
 6. A classifier as claimed in claim 5, wherein the periphery of the drum like means is comprised of spaced ribs parallel to the axis of the drum.
 7. A classifier as claimed in claim 5, wherein the drum-like means comprises spaced ribs parallel to the axis of the drum each rib having an isosceles angular shape in cross section pointing in the direction of rotation of the drum. 